a. Field of the Invention
This invention relates to methods and apparatuses for providing a fusible link for electrical discharge reactors, particularly dielectric barrier discharge reactors.
b. Description of the Related Art
Electrical discharge reactors are used for many purposes, including the production of ozone and ultraviolet light, and for the destruction of noxious substances in gaseous emissions. A dielectric barrier discharge (“DBD”) reactor may comprise multiple tubes configured in an array similar to a tube and shell heat exchanger. DBD is synonymously known as silent discharge or non-thermal plasma discharge. In such a multi-tube DBD reactor, a high voltage electrode is centered within a ground tube creating an annulus or air gap between the electrode and ground tube. When the electrodes are energized, a barrier discharge is generated by the application of a voltage potential between a high voltage source through at least one dielectric barrier, including the air gap to the ground tube that is at ground potential.
In a reactor for destroying pollutants, gases containing pollutants are passed through this air gap and are oxidized by the discharge that creates oxygen and hydroxyl radicals. The ground tubes of such a DBD reactor also act as a heat exchanger since some of the energy generated by a barrier discharge is converted to heat and therefore can be dissipated through the ground tubes. For this intended application, DBD reactors normally operate at voltages up to about 60 kV AC peak-to-peak at a frequency between about 1.6 kHz and 2.4 kHz.
DBD reactors in industrial applications have a large number of electrodes and ground tubes so that they can treat all the fluid from an industrial process, for example, flue gas from the boiler of a fossil fueled power plant. A plurality of reactors may be used, and may be provided in modules to facilitate their manufacture and installation. All the electrodes of at least one module are preferably wired in parallel to a power supply to minimize the number of power supplies necessary. The preferred embodiment is a DBD reactor that multiple electrodes wired in parallel. The term “power supply” as used herein refers to a device that converts electricity found in standard industrial electrical systems to the voltage and frequency required to energize discharge reactor electrodes as well as provide various key operating control functions.
During normal reactor operation, dielectric failures can occur. These failures are generally the result of localized heating of the dielectric material, which in the preferred embodiment is fused silica, or quartz. Other materials having a high dielectric constant can be also used, which is readily appreciated by those having skill in the art. Since the dielectric strength of a material is temperature dependent, excessive heating of the dielectric can reduce the dielectric strength of the material to a point where the material can no longer stand off the voltage necessary to maintain a barrier discharge. This condition is known as thermal dielectric breakdown. The result of a dielectric breakdown is usually a puncture through the dielectric material which provides a direct electrical path to ground. This direct path to ground becomes the path of least resistance creating a short circuit, which results in all of the energy from the power supply being delivered to the failed electrode. The energy transferred to the short circuit can be sufficient to result in additional melting of the dielectric and/or puncture of the ground tube.
Prior to the use of the present invention, when a dielectric failure occurred, the power supply was required to be de-energized once the short circuit was detected by the power supply controller. This feature is typically built into the power supply controller to prevent damage to the power supply and/or reactor components. After a short circuit was detected, it was then necessary to bring the unit off line. Personnel would then lockout all energy sources, open up the reactor housing, test the air quality for suitability for human occupation, and search for the failed electrode. This necessary exercise usually resulted in at least a day of lost operations when required cool-down and heat-up cycles are taken into consideration.
Many industrial processes can use the benefits of electrical discharge reactors. However, many industrial processes require consistent, continuous operation. For example, de-energizing the DBD reactors for any length of time in a fossil fueled power plant would also require shutting down the boiler, resulting in a substantial loss of electricity generation revenue.
Electrical discharge reactors are often used in ozone generators. Ozone generators, which typically operate at 15 kV peak-to-peak, have used fuses to isolate a particular failed electrode tube. An example of such a fuse is found in U.S. Pat. No. 4,293,775 that issued on Oct. 6, 1981, to Feuerstake et al. Feuerstake teaches, among other things, that prior art fuses sometimes blow when no tubes have failed. In addition, sometimes electrode tubes fail and none of the fuses blow. Feurerstake's solution was to add a resistance element in series with the fuse element. This may have been an acceptable solution for electrical discharge reactors operating at 15 kV peak-to-peak, but it has been found not to work in reactors that operate at much higher voltages such as 60 kV peak-to-peak. The problem is that the electrical resistance of the air breaks down, and an arc crosses the gap between the space once occupied by the fuse material.
Reactor fuses may have to operate in a corrosive environment that may tend to either prevent a fuse from blowing or causing a fuse to blow prematurely. This problem was addressed in U.S. Pat. No. 4,296,397 that issued on Oct. 20, 1981, to Sedberry. Sedberry's solutions were primarily applicable to ozone generators, because it is known that ozone can be corrosive. Sedberry's solutions were to use an ozone corrosion-resistant fuse wire, and to encapsulate the fuse in a closed, tubular housing. Sedberry also mentioned the added benefit that rigid insulator spacing, terminals, and fuse wire can be provided as a unitary structure. However, like Feuerstake, Sedberry discloses that ozonators operate at lower voltages than that used for pollution control, reciting a range between 6 kV peak-to-peak and 22 kV peak-to-peak. There is no mention of the suitability of his invention for other corrosive environments, including NOx, SO2, acids, and fine particles.
What is needed, therefore, is a fuse means to isolate an individual electrode from the power supply circuit that operates at high voltage to permit the remaining electrodes to operate unimpeded. With the use of such fuse means, electrode maintenance can be delayed until the reactor is taken off line for maintenance during a scheduled outage. It is also beneficial to ensure that the appearance of the fuse from a failed electrode be significantly different from that of an intact fuse in order to aid the maintenance personnel in locating and replacing the failed components. Therefore, it is advantageous if the fuse assembly is a unitary structure, a failed fuse assembly is easily discernible from intact fuse assemblies, and is unaffected by corrosive environments.